The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of oxygen-containing (by way of example methanol), halogenide-containing or sulphur-containing organic compounds to hydrocarbons and especially light olefins (by light olefins it is meant C2 to C4 olefins) or gasoline and aromatics. In the present application the conversion of said oxygen-containing (also referred to as oxygenates), halogenide-containing or sulphur-containing organic compounds to hydrocarbons and especially light olefins is referred to as the XTO process. The interest in the XTO process is based on the fact that feedstocks, especially methanol can be obtained from coal, biomass, organic waste or natural gas by the production of synthesis gas which is then processed to produce methanol. The XTO process can be combined with an OCP (olefins cracking process) process to increase production of olefins. The XTO process produces light olefins such as ethylene and propylene as well as heavy hydrocarbons such as butenes and above. These heavy hydrocarbons are cracked in an OCP process to give mainly ethylene and propylene.
U.S. Pat. No. 6,951,830B2 relates to a catalyst composition, a method of making the same and its use in the conversion of a feedstock, preferably an oxygenated feedstock, into one or more olefin(s), preferably ethylene and/or propylene. The catalyst composition comprises a molecular sieve, such as a silicoaluminophosphate and/or an aluminophosphate, hydrotalcite, and optionally a rare earth metal component. The rare earth metal compound can be in the form of acetates, halides, oxides, oxyhalides, hydroxides, sulphides, sulphonates, borides, borates, carbonates, nitrates, carboxylates and mixtures thereof.
US20070043250A1 describes an oxygenate conversion catalyst useful in the conversion of oxygenates such as methanol to olefinic products which is improved by the use of a catalyst combination based on a molecular sieve in combination with a co-catalyst comprising a mixed metal oxide composition which has oxidation/reduction functionality under the conditions of the conversion. This metal oxide co-catalyst component will comprise a mixed oxide of one or more, preferably at least two, transition metals, usually of Series 4, 5 or 6 of the Periodic Table, with the metals of Series 4 being preferred, as an essential component of the mixed oxide composition. The preferred transition metals are those of Groups 5, especially titanium and vanadium, Group 6, especially chromium or molybdenum, Group 7, especially manganese and Group 8, especially cobalt or nickel. Other metal oxides may also be present. The preferred molecular sieve components in these catalysts are the high silica zeolites and the silicoaluminophosphates (SAPOs), especially the small pore SAPOs (8-membered rings), such as SAPO-34. These catalyst combinations exhibiting reduced coke selectivity have the potential of achieving extended catalyst life. In addition, these catalysts have the capability of selectively converting the hydrogen produced during the conversion to liquid products, mainly water, reducing the demand on reactor volume and product handling.
U.S. Pat. No. 7,186,875 discloses a process for converting an oxygenate-containing feedstock into one or more olefins in a reactor system including a plurality of fixed bed reactors each containing a catalyst composition comprising a molecular sieve and at least one metal oxide having an uptake of carbon dioxide at 100° C. of at least 0.03 mg/m2 of the metal oxide. Each reactor is sequentially rotated between at least one operating mode, wherein the catalyst composition in the reactor is contacted with the oxygenate-containing feedstock, and a regeneration mode, wherein the catalyst composition in the reactor is contacted with a regeneration medium. The molecular sieve is a silicoaluminophosphate (SAPO) and/or a metal substituted SAPO. The metal oxide used in the composition, it is stated, is different from typically used binders and/or matrix material, in that it extends the life of the catalyst composition. Suitable metal oxides include those metal oxides having a Group 2, Group 3 (including the Lanthanides and Actinides) or Group 4 metal. There is no mention of any metal salts.
US 2003/0176752 describes a catalyst composition, a method making the same and its use in the conversion of a feedstock, preferably an oxygenated feedstock, into one or more olefin(s), preferably ethylene and/or propylene. The catalyst composition comprises a molecular sieve and at least one oxide of a metal from Group 4, optionally in combination with at least one metal from Groups 2 and 3. The metal oxide has an uptake of carbon dioxide at 100° C. of at least 0.03 mg/m3. The molecular sieve is preferably a silicoaluminophosphate and/or metal derivatives thereof.
US 2004/0030213 discloses describes an oxygenate conversion catalyst based a combination of a molecular sieve such as SAPO-34 and an oxide of a metal of Group 3, including the lanthanide series and the actinide series. Examples of such oxides include lanthanum oxide, yttrium oxide, scandium oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide and thorium oxide.
This catalytic combination is reported to result in similar advantages when used in methanol conversion reactions and, in addition, results in a reduction in the amounts of undesirable by-products such as aldehydes and ketones, especially acetaldehyde. In addition, it is claimed that the catalyst compositions are less susceptible to coke formation and thus have longer lifetimes. It is also stated that the higher density of these catalyst compositions is believed to improve operability in the overall conversion process. The denser catalyst particles are retained to a greater extent within the unit, whether in the reactor or its associated regenerator, resulting in lower catalyst losses.
WO 1998029370 discloses the conversion of oxygenates to olefins over a small pore (less than 5 nm) non-zeolitic molecular sieve containing an oxide of a lanthanide metal or an actinide metal, scandium, yttrium, a Group 4 metal, a Group 5 metal or combinations thereof. The metal-containing compound is introduced to the non-zeolitic molecular sieve in the form of the corresponding halide, sulphate, acetate, formate, propionate, oxalate, maleate, fumarate, carboxylate, alkoxide, carbonyl, nitrate or mixtures thereof. These small pore molecular sieve compositions are claimed to be more stable even at high conversion rates. These salts however are oxide-precursor salts and thus not stable at the high temperature conditions of the MTO process. Furthermore, selectivity to propylene is markedly low.
Molecular sieves in combination with matrix and binder components for XTO processes are known in the art. Usually, the binder and matrix are chemically neutral materials, typically serving only to provide desired physical characteristics to the catalyst composition. Usually, they have very little effect on catalytic performance. These molecular sieve catalyst compositions are formed by combining the molecular sieve and the matrix e.g. an inorganic oxide such as alumina, titania, zirconia, silica or silica-alumina with a binder, e.g. clay, to form a cohesive, mechanically stable, attrition-resistant composite of the sieve, matrix material and binder. In particular, the use of silica (SiO2) as a binder/matrix material is well known in the art. This solid is neutral and is selected when catalytic effects of the binder/matrix are undesired. Typically, rare earth elements, which are very expensive, are used in such catalyst composites.
Metal is introduced typically in the form of oxides/oxide-precursor salts by ion-exchange or impregnation. However, ion-exchange/impregnation potentially leads to the modification of the acidity of catalytic sites throughout the whole microporous structure of the molecular sieve. This could lead to decreased catalytic activity. Metal oxides are chemically active compounds. Without taking special precautions during pre-treatment and catalyst formulation these compounds may partially damage the molecular sieve pore structure. The proposed present invention is very different from the prior art. It avoids the use of metal oxides or unstable oxide-precursor salts. The combination of molecular sieves with chemically inert metal salts which are stable under the conversion process of the oxygenates to the olefins, allows selectively modifying only the sites located on the external surface and in the pore mouths of the molecular sieve. As a result, the formation of side products is minimised and coke formation is decreased without losses in the catalyst's activity.
Small pore silicoaluminophosphate (SAPO) molecular sieve catalysts have excellent selectivity in oxygenates to light olefin reactions. However, these catalysts have a tendency to deactivate rapidly during the conversion of oxygenates to olefins and the ratio C3/C2 could be improved. Therefore a need exists for methods to decrease the rate of deactivation of small pore molecular sieve catalysts during such conversions and to improve the yield of light olefins and the C3/C2 ratio.
It has been discovered that addition of a small amount of metal salts to a small pore MeAPO molecular sieve or optionally to a composite molecular sieve containing a combination of small pore MeAPO molecular sieve with medium or large pore crystalline silicoaluminate, silicoaluminophosphate materials or silicoaluminate mesoporous molecular sieves leads to substantial increase of C3/C2 ratio, yield of light olefins and stability in XTO than was obtained over the parent molecular sieve alone (MeAPO).
Higher stability of the blended catalysts together with the metal salt provides a possibility to operate at higher flow rate, increase the catalyst on-stream time in XTO conversion reactor and decrease the size of regeneration section or the frequency of regeneration. (on-stream time is the time that a catalyst resides in the conversion reactor and exhibits still sufficient catalytic activity, before it has to be taken off-line for regeneration or replacement)
Unexpectedly, this catalyst composite possesses reduced coke selectivity in comparison with the weighted average of the individual molecular sieves.
The excess of C4+ as well as ethylene can be converted to propylene in an olefin cracking fixed bed reactor (OCP) in combination with the XTO process. Ethylene can be recycled back in XTO reactor or to the OCP reactor. The excess C4+ as well as the ethylene can be converted to more propylene by recycling C4+ and ethylene back to the XTO reactor. The catalyst blend allows the conversion of organic compounds, C4+ and ethylene at the same time.
Stated above, small pore MeAPO molecular sieves contain 8-membered rings as the largest pore aperture in the structure, medium pore crystalline silicoaluminates contain 10-membered rings as the largest pore aperture; large pore crystalline silicoaluminates contain 12-membered rings as the largest pore aperture. Stated above medium, large pore and mesoporous molecular sieves have acidic properties, which are capable in catalysing the formation of aromatic precursors from used feedstock.
In the XTO process the ethylene, propylene and higher hydrocarbons are formed via a “carbon pool” mechanism (Dahl and Kolboe 1994 Journal of Catalysis 149(2): 458-464; Dahl and Kolboe 1996 Journal of Catalysis 161(1): 304-309; Stocker 1999, Microporous and Mesoporous Materials 29(1-2): 3-48). Ethylene, propylene and C4+ olefins selectivities in XTO process are related to the number of methyl groups attached to benzene rings trapped in the nanocages. The product spectrum varies strongly with the pore size of the catalytic material (shape selectivity), and when the small pore SAPO-34 (chabasite structure) is used as catalyst the hydrocarbon products are mostly ethene and propene, and some substantially linear butenes, the only product molecules small enough to escape with ease through the narrow pores.
It has been discovered that medium or large pore crystalline silicoaluminate, silicoaluminophosphate materials or silicoaluminate mesoporous molecular sieves play a role of a faster in-situ supply for aromatic precursors for olefin production by the carbon pool mechanism. One optional aspect of this invention is in-situ on-purpose formation of some additional organic reaction centers by adding to the MeAPO a small amount of acid co-catalyst with larger pore opening than the MeAPO. These materials are capable to produce a small amount of higher molecular weight precursors that can enter into the pore system of the small pore MeAPO where they are converted into the aromatics under XTO conditions. These aromatics constitute the active centers for XTO according to the carbon pool mechanism. These aromatics are trapped by MeAPO micro porous system in a more optimum way without formation of a lot of coke by-products. This allows an increased catalyst stability and C3/C2 ratio.
Without being bonded by any theory, inventors think that an optimum concentration of the methylbenzenes organic reaction centers leads to higher light olefins production and to a slower deactivation. However the olefin production is limited by diffusion of heavy olefins out of the micropore system of MeAPO in which usually methylbenzenes are trapped. Formation of the methylbenzenes inside of MeAPO pore system requires a certain time and is accompanied by coke formation. More coke formation in the small pore MeAPO reduces the accessible pore volume and results in faster loss of catalytic activity.